The present invention relates to an ink supply amount adjustment method and apparatus for a printing press, which adjust the ink supply amount to a printing plate on the basis of the density value or color value of a printing product.
FIG. 28 shows the main part of the inking device (inker) in each color printing unit of a web offset printing press. Referring to FIG. 28, reference numeral 1 denotes an ink fountain; 2, ink stored in the ink fountain 1; 3, an ink fountain roller; 4-1 to 4-n, a plurality of ink keys juxtaposed in the axial direction of the ink fountain roller 3; 5, an ink ductor roller; 6, ink rollers; and 7, a printing plate mounted on a plate cylinder 8. An image is printed on the printing plate 7.
In this inking device, the ink 2 in the ink fountain 1 is supplied to the ink fountain roller 3 through the gap between the ink keys 4-1 to 4-n and the ink fountain roller 3. The ink supplied to the ink fountain roller 3 is supplied to the printing plate 7 through the ink rollers 6 by the duct operation of the ink ductor roller 5. The ink supplied to the printing plate 7 is printed on a printing paper sheet through a blanket cylinder (not shown).
FIG. 2 shows a printing product printed by this printing press. A band-shaped color bar 9-2 is printed on the margin portion of a printing product 9 except an image region 9-1. In general four-color printing, the color bar 9-2 includes regions S1 to Sn including density measurement patches (solid patches at a percent dot area of 100%) 9a1, 9a2, 9a3, and 9a4 of black, cyan, magenta, and yellow. The regions S1 to Sn correspond to the key zones of ink keys 4-1 to 4-n-1 to 4-n in each color printing unit of the printing press.
[Color Matching]
A reference density value is set in advance for each color printing unit. More specifically, a reference density value is set in advance for each of black, cyan, magenta, and yellow. In printing the printing product 9, a color matching operation is done to make the density value of each color coincide with the reference density value. This color matching operation is executed by the ink supply amount adjustment apparatus before final printing (at the time of preparation for printing) or during final printing on the basis of the density of a density measurement patch 9a (9a1, 9a2, 9a3, or 9a4) of each color in the color bar 9-2 printed on the printing product 9.
For example, the region S1 in the printing product 9 will be described as a representative. The density value of the density measurement patch 9a of each color on the printing product 9, which is extracted before or during final printing, is measured. The difference between the measured density value of each color and the preset reference density value of each color is obtained. The adjustment amounts of the opening ratios of the ink keys 4-1 to 4-n-1 (the adjustment amounts of ink supply amounts to the region S1) in each color printing unit are obtained from the obtained density difference of each color. The obtained adjustment amounts (reference correction amounts) are multiplied by a unique coefficient (control ratio) to obtain a correction amount. The correction amount is fed back to adjust the opening ratios of the ink keys 4-1 to 4-n-1 in each color printing unit.
In a similar way, for regions S2 to Sn as well, the adjustment amounts of the opening ratios of the ink keys 4-1 to 4-n-2 to 4-n (the adjustment amounts of ink supply amounts to the regions S2 to Sn) in each color printing unit are obtained. The obtained adjustment amounts (reference correction amounts) are multiplied by a control ratio to obtain a correction amount. The correction amount is fed back to adjust the opening ratios of the ink keys 4-1 to 4-n-2 to 4-n in each color printing unit.
In adjusting the opening ratios of the ink keys 4-1 to 4-n in each color printing unit, the only allowable density difference of each color is defined for the density difference (measured density difference) between the measured density value of each color (measured density difference) and the preset reference density value of each color. The ink supply amount is adjusted for only colors whose measured density differences are larger than the allowable density difference (Japanese Patent Laid-Open No. 2003-118077).
In the above-described conventional ink supply amount adjustment method, however, only one kind of allowable density difference is defined for the measured density difference of each color. This causes the following problems.
For example, assume that the allowable density difference is small, and the ink supply amount adjustment interval (printing product sampling interval) is short. In this case, before the influence of the precedingly adjusted ink supply amount is sufficiently reflected on the printing product, the next ink supply amount adjustment is done. Accordingly, a hunting phenomenon (the color tone becomes unstable because of the variation in color density on the printing product) of the ink thickness on the paper sheet occurs.
In addition, the density value of a specific part of the printing product may temporarily largely vary due to an unexpected accident (paper fold error, paper breakage, or smear) on the printing product. In this case, the ink supply amount is adjusted to a value largely shifted from what the ink supply amount should be. As a result, the amount of wasted paper increases.